The composition of Ringer solution, a crystalloid fluid that is often used in anaesthesia and intensive care, varies depending on the manufacturer. The knowledge of the actual content in electrolytes and of the characteristics of this fluid is necessary before it is used. We call attention to a certain Ringer solution (Ringer Maco Pharma, Maco Pharma), for which the manufacturer's information about the tonicity and the osmolarity was incorrect. Contrary to what is written on the bag and in the product description (isotonicity, osmolarity of 276.8 mOsm/l), the theoretical osmolarity was 221.4 mOsm/l and the measured osmolality was about 208 mmol/kg, exposing the hypotonic characteristics of this fluid. The use of this product is potentially dangerous in patients with pathologies where the infusion of free water is especially badly supported.